Chemical-mechanical planarization or chemical-mechanical polishing, commonly abbreviated CMP, is a technique used in semiconductor fabrication for planarizing a semiconductor wafer or other substrate.
The process uses an abrasive and corrosive chemical slurry (commonly a colloid) in conjunction with a polishing pad and retaining ring, typically of a greater diameter than the wafer. The pad and wafer are pressed together by a dynamic polishing head and held in place by a plastic retaining ring. The dynamic polishing head is rotated with different axes of rotation (i.e., not concentric). This removes material and tends to even out any irregular topography, making the wafer flat or planar. This may be necessary in order to set up the wafer for the formation of additional circuit elements. For example, this might be necessary in order to bring the entire surface within the depth of field of a photolithography system, or to selectively remove material based on its position. Typical depth-of-field requirements are down to Angstrom levels for the latest 45 nm technology node.
The process of material removal is not simply that of abrasive scraping, like sandpaper on wood. The chemicals in the slurry also react with and/or weaken the material to be removed. The abrasive accelerates this weakening process and the polishing pad helps to wipe the reacted materials from the surface. The process has been likened to that of a child eating a gummy candy. If the candy sits on the tongue without being scraped around, the candy becomes covered with a gel coating, but the majority of the candy is not affected. Only with a vigorous scraping does the candy dissolve away. Another analogy is the act of brushing one's teeth. The toothbrush is the mechanical part and the toothpaste is the chemical part. Using either the toothbrush or the toothpaste alone will get one's teeth somewhat clean, but using the toothbrush and toothpaste together makes a superior process.
Accordingly, in addition to advances in slurry technology, the polishing pad plays a significant role in increasingly complex CMP operations. However, additional improvements are needed in the evolution of CMP pad technology.